1. Field of the Invention
The present invention relates to a high efficiency light emitting device, and more particularly to a III-V compound semiconductor light emitting diode with a highly reflective metal reflector therein to avoid the light absorption by the substrate.
2. Description of the Prior Art
The conventional AlGaInP LED, as shown in FIG. 1, has a double heterostructure (DH), which consisted of an n-type (AlxGa1xe2x88x92x)0.5In0.5P lower cladding layer 3 with an Al composition of about 70%-100%, formed on an n-type GaAs substrate 1, an (AlxGa1xe2x88x92x)0.5In0.5P active layer 5 with an Al composition of 0%-45%, a p-type (AlxGa1xe2x88x92x)0.5In0.5P upper cladding layer 7 with an Al composition 70%-100% and a p-type high energy bandgap current spreading layer 9 such as layers of GaP, GaAsP, AlGaAs, or ZnO. However, the portion of the light emits from the active layer 5 towards the substrate will be totally absorbed by GaAs substrate 1. Therefore, the external quantum efficiency of this kind of conventional AlGaInP LED is small. Besides, the thermal conductivity of GaAs is only about 44 W/m-xc2x0 C. The low thermal conductivity of the GaAs substrate 1 is not good enough to dissipate the heat generated.
To overcome the substrate absorption problem, several conventional LED fabrication technologies have been disclosed. However, those conventional technologies still have several disadvantages and limitations. For example, Sugawara et al. disclosed a method published in Appl. Phys. Lett. Vol. 61, 1775-1777 (1992), The LED structure is similar to the FIG. 1, thus, in FIG. 2, the similar function layers are labeled with the same reference numerals. Sugawara et al. added a distributed Bragg reflector (DBR) layer 2 in between the GaAs substrate 1 and lower cladding layer 3 so as to reflect those light emitted toward the GaAs substrate 1, as shown in FIG. 2. Further they added a blocking layer 10 to enhance current spread. However, the maximum reflectivity of the DBR layer 2 used in AlGaInP LED is only about 80% and the reflectivity thereof also depends on the reflection angle. The DBR layer 2 can only effectively reflect the light vertically emitted towards the GaAs substrate 1, so that the improvement of external quantum efficiency is limited.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1xe2x88x92x)0.5In0.5P/GaP light emitting diode [Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994); Very high efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1xe2x88x92x)0.5In0.5P/GaP]. As shown in FIG. 3, a transparent-substrate 13 (TS) is replaced for the GaAs absorption substrate (not shown). The TS AlGaInP LED was fabricated by growing a very thick (about 50 um) p-type GaP window layer 11 formed atop epi-layers light emitting structure 12 (0.75 xcexcm p-type cladding layer 3 of Al0.5In0.5P/active layer 5 of (AlxGa1xe2x88x92x)0.5In0.5P/1 xcexcm n-type cladding layer 7 of Al0.5In0.5P with GaAs as temporary substrate) by using hydride vapor phase epitaxy (HVPE). Subsequently, the temporary n-type GaAs substrate was selectively removed using conventional chemical etching techniques. After removing the GaAs substrate, the LED epilayer structure 12 is then bonded to an 8-10 mil thick n-type GaP substrate 13. The resulting TS AlGaInP LED exhibits two times improvement in light output compared to absorbing substrate (AS) AlGaInP LEDs. However, the fabrication process of the TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Another conventional technique is shown in FIG. 4. The schematic diagram, which is proposed by Haitz et al., in U.S. Pat. No. 5,917,202. The light emitting diode epi-layers 40 included active layer 41 and n-type GaP 45p and P-type GaP 43, are prepared. Thereafter a reflective metal layer 47 with zinc, germanium or the like doped and an n-type electrode 47a, which can be made from the same material as reflective metal layer 47, is formed on both the upper and bottom surface. After that, a pulse laser beam is then utilized to heat some predetermined spots so as to form alloy spots 49 by reacting the reflective metal layer 47 with the p-type GaP 43, and reacting the n-type electrode 17a with the n-type GaP 45. The alloy spots 49 are in a form of grid pattern. The pitch between two neighbor alloy spots 49 and the spot size itself rely on the current effectively spreading area from each alloy spot 49.
In terms of alloy spots, ohmic contacts are formed. The process skips a high temperature thermal anneal in the furnace. However, the product yield depends on the burn-in scan rate by the laser beam. And hence, the product rate is limited. Moreover, any high temperature process in the post-process is still inhibited.
Another embodiment of Haitz""s is shown in FIG. 5. The processes are as follows. The light emitting epi-layers are first adhered to a transparent substrate 52. Next, a dielectric layer 53 is formed on the transparent 52. Afterward, a lithographic and an etch process are successively carried out to form a plurality of contact channels 54. An ohmic contact metal refilled process is then followed. The contact channels make the current flow from the n-electrode to the p-electrode without being interrupted. However, the processes are still complicated. Furthermore, it degrades the performance under high current flow and any thermal process because the dielectric layer 52 is an insulator for both heat and electrical-conductive properties.
An object of the present is thus to provide a method of making a light emitting diode with a high efficient reflective metal.
The present invention disclosed a method of fabricating a light emitting diode with a high efficient reflective metal layer. To prevent the reflective metal layer from reacting with the epi-LED layer structure during a thermal process, a transparent electrical-conductive oxide layer such as a layer of In2O3, SnO2, CdO, ZnO, ITO, CTO, CuAlO2, CuGaO2, or SrCu2O2 is formed in between them. A reflective metal formed on the transparent electrical-conductive oxide layer is then followed. After that, a silicon-base substrate is bonded to the reflective metal layer through a metal bonding layer. Finally, a removal of the temporary substrate from the LED epi-layers, a step of formation an n-electrode and an annealing process are sequentially carried out.
Four preferred embodiments are proposed to improve the ohmic contact between the ITO layer and epi-LED layers.
In the first preferred embodiment, an ohmic contact grid pattern is formed in the transparent electrical-conductive oxide layer and is adjacent to the interface between the transparent electrical-conductive oxide layer and the p-type ohmic contact layer of the LED epi-layers.
In the second preferred embodiment, a thin film layer selected from III-V group compound, of which energy bandgap is small, is formed on the ohmic contact layer of the LED epi-layers before step of forming transparent electrical-conductive oxide layer to improve the ohmic contact between the ITO layer and epi-LED layers. Afterwards, the process steps are as depicted before,
In the third preferred embodiment according to the present invention, which modified from the second preferred embodiment a thin transparent metal layer is replaced for the low bandgap III-V group compound semiconductor layer.
In the fourth preferred embodiment, a plurality of ohmic contact channels is formed in the ITO layer to play the role of the ohmic contact improvement between the ITO layer and epi-LED layers.